How would I go about making these seem more like planets?
For the Internal renderer:



Add a direction light and give your sphere a new material. Turn the Specular -> Intensity down to 0.


And add your texture. Result:



For the Cycles renderer:



Add a directional light. And give the material a diffuse shader like this:


In the Image Texture node you can put your texture. (Don't forget to UV unwrap your spheres)
This is the result without a texture: